Theorem B.1 (Theorems 15.13 and 15.14 in [1]}) 

Consider direction \(u\)  with directional coordinates \(a=(a_1, a_2, a_3, a_4)\) . Then for any trivariate polynomial \(p\) 
with B-form in (REF ), the directional derivative is
\(D_{u}p(v)= d \sum _{i+j+k+l=d-1} \gamma _{ijkl}^{(1)}(a) B_{ijkl}^{d-1}(v),\) 
